There are a variety of situations which require belowground communications. For example, miners in a coal mine or utility workers in a utility tunnel may require the ability to communicate with each other, as well with aboveground personnel. Another situation would be in the context of underground transportation, such as subways.
Given the potential danger involved in such underground situations, reliable channels of communications with aboveground personnel is particularly important. The current approach to provide communication to such underground areas is the use of a so-called leaky coax cable. Leaky coax cable systems consist of a cable run along predefined belowground area which emits and receives radio waves. Leaky coax cable systems have a limited range due to the high radio frequencies used. Such high frequency transmissions do not readily penetrate solid rock, which limits such systems to essentially being line-of-sight communication systems.
In emergency situations, such as an explosion or cave in, communication systems, such as leaky coax cable systems, are susceptible to being damaged. In addition, such hardwired systems are particularly susceptible to being run over by the heavy equipment moving in and out of mines. This can be a life threatening situation, preventing those underground from being directed to a safe exit or located for rescue.
Solutions have been demonstrated for establishing wireless, 2-way communications through earth and rock, such as the systems disclosed and claimed in co-pending U.S. patent application Ser. Nos. 11/773,847 and 11/773,814, which are hereby fully incorporated by reference. However, such systems may also be susceptible to malfunction, interference or otherwise being compromised.
As such, there has been an increased interest in providing a redundant HASC communication solution, specifically for underground communication applications, which provides reliable communication during both normal operation, as well as during emergency situations or network outages.